Conventionally, as a distributed power supply system, an in-house power generation system disclosed in Patent Literature 1 is proposed. FIG. 10 is a bock diagram showing a configuration of a conventional distributed power supply system disclosed in Patent Literature 1.
As shown in FIG. 10, a distributed power supply system 103 includes a fuel cell apparatus 104 as a distributed power supply apparatus which is interactively connected (interconnected) to commercial power utilities 101 via electric wires (cables). The commercial power utility 101 is a single-phase three-wire AC power supply composed of U-phase, O-phase, and W-phase. Branch wires are extended from the electric wires respectively corresponding to U-phase, O-phase, and W-phase, and are connected to a customer load 102. The distributed power supply system 103 includes current sensors 105a, 105b for detecting magnitudes and directions (positive and opposite (negative) directions)) of an AC current flowing through the U-phase and an AC current flowing through the W-phase, a voltage sensor 106 for detecting a voltage of the commercial power utility 101, and a heater 107 which is an internal load in the system 103. The distributed power supply system 103 further includes a controller 108 for controlling the operation of the system 103, a power integration meter 109 for integrating electric power consumed in the customer load 102 and in the system 103, and a LCD (liquid crystal display) 110 which is a display means and notification means for displaying information such as an electric power value and an abnormal state of the system 103.
In Patent Literature 1, as an installation direction of the current sensors 105a, 105b, for example, an installation direction in which a current flowing from the commercial power utility 101 toward the fuel cell apparatus 104 is detected as a positive current is referred to as a positive installation direction.
The controller 108 includes a power calculating section 111, a current sensor installation direction determiner section 112, a nonvolatile memory 113, a sign inverting section 114, and a heater control section 115. The power calculating section 111 calculates the electric power consumed in the U-phase based on a detected value of the current sensor 105a and the electric power consumed in the W-phase based on a detected value of the current sensor 105b. The current sensor installation direction determiner section 112 determines the installation directions of the current sensors 105a, 105b with respect to the electric wires. The nonvolatile memory 113 is a memory means for storing data of result of determination performed by the current sensor installation direction determiner section 112. The sign inverting section 114 properly corrects positive/negative signs of values of electric power consumption calculated by the power calculating section 111 based on information relating to the installation directions of the current sensors 105a, 105b stored in the nonvolatile memory 113. The heater control section 115 controls the electric power supplied to the heater 107.
In the distributed power supply system 103, during an ON-state of a power supply, the controller 108 determines the installation directions of the current sensors 105a, 105b. More specifically, the controller 108 causes the heater control section 115 to turn ON the heater 107 in a state where the fuel cell apparatus 104 is generating no electric power, to supply the electric power from the commercial power utility 101 to the fuel cell apparatus 104 via the electric wires. As a result, the fuel cell apparatus 104 consumes the electric power. Therefore, the directions of the currents detected by the current sensors 105a, 105b should be decided as particular directions, and the electric power consumed by the customer load increases temporarily. During this time (during a state in which the heater 107 is ON), the current sensors 105a, 105b obtain current values, and at the same time, the voltage sensor 106 obtains voltage values. The power calculating section 111 calculates the electric power value at the U-phase and the electric power value at the W-phase based on the current values and the voltage values.
Based on the calculated electric power values, the current sensor installation direction determiner section 112 determines the installation directions of the current sensors 105a, 105b, and the nonvolatile memory 113 stores data of results of determination corresponding to the U-phase and the W-phase, respectively. This determination is performed as follows.
During the state where the fuel cell apparatus 104 is generating no electric power, as described above, by turning ON the heater 107, the electric power is supplied from the commercial power utility 101 to the fuel cell apparatus 104. If the installation direction of the current sensor 105a(105b) is correct, a positive current value is detected, and the electric power value calculated by the power calculating section 111 is a positive value. Therefore, in determination as to the installation direction, if the electric power value calculated by the power calculating section 111 is not less than a predetermined value (e.g., 0 W), it can be determined that the current sensor 105a (105b) is installed in a positive direction, while if the electric power value calculated by the power calculating section 111 is less than the predetermined value, it can be determined that the current sensor 105a (105b) is installed in an opposite (negative) direction. Determination information (installation direction information) corresponding to each of the U-phase and the W-phase, for example, the positive direction or the opposite direction, is stored in the nonvolatile memory 113.
After the determination information is obtained and stored, the controller 108 causes the heater control section 115 to turn OFF the heater 107, and thus terminates determination as to the installation direction of the current sensor 105a (105b). Thereafter, the controller 108 causes the sign inverting section 114 to correct (invert the sign) the electric power value calculated for each of the U-phase and the W-phase based on the installation direction information of the current sensor 105a(105b). The power integration meter 109 integrates the corrected electric power value and the LCD 110 displays and outputs the integrated electric power value.
In general, each of the current sensors 105a, 105b has a region in which a relationship between a current value which is a detected target and an output voltage value of the sensor is linear and a region in which the relationship is non-linear. FIG. 11 is a graph showing a relationship between a detected current value and an output voltage value in a current sensor applied to, for example, the conventional distributed power supply system disclosed in Patent Literature 1. As shown in FIG. 11, the current sensor has a characteristic in which the output voltage value changes linearly with respect to the detected current value in a region in which the current value is less than a predetermined value and changes non-linearly in a region in which the current value is not less than the predetermined value. In other words, the current sensor functions as a linear sensor when the detected current value is less than the predetermined value, whereas the current sensor does not function as the linear sensor when the detected current value is not less than the predetermined value, in which state, the output voltage value is substantially constant regardless of a change in the current value, and the current sensor is unable to measure the current value.
In particular, the electric power consumed by the customer load 102 is not constant but changes significantly all the time. Therefore, in some cases, the current value changes beyond a limit value of the current sensor 105a (105b) as a liner sensor. In this case, detecting accuracy of the current value decreases significantly. In this way, the current sensor has a range of a detected current value in which accuracy is guaranteed and the above described predetermined value is its upper limit value (detected upper limit value).    Patent Literature 1: Japanese Laid-Open Patent Application Publication No. 2009-118673